Page 140 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 6. Roles of thermokarst lakes in a warming world
Figure 2. Scheme illustrating the thermokarst lake microbial carbon cycle within ice-rich Yedoma deposits that are characterized by ice wedges. The microbial community within the unfrozen ground layer (Talik) degrades organic matter (OM) to CH4 through fermentative and syntrophic processes (1- 4). CH4 can be oxidized to CO2 by aerobic and anaerobic methanotrophs (5). The non-oxidized fraction of CH4 can escape via diffusion, ebullition and the release of ice-bubble storage (6-8).
One interesting aspect of thermokarst GHG emissions is that CO2 and CH4 production respond differently to warming. A study of 40 Alaskan lakes across a longitudinal transect that includes lakes formed in continuous, discontinuous, and isolated permafrost showed that all were net sources of CO2 and CH4, but that the warming impact on CH4 production was twice as high compared with CO2 (Sepulveda-Jauregui et al. 2015). Metje and Frenzel showed highest methanogenesis rates between 26 and 28°C, and 17% of the maximum activity at 4°C (Metje and Frenzel 2007). Little is known about the effects of seasonality, and only few studies have addressed year-round measurements in the field, due to logistic and sampling limitations. It has been shown that floating ice lakes have a year-round active layer and that CH4 is trapped in ice bubbles during winter and released in spring, causing seasonal peak emissions (Arp et al. 2012; Sepulveda-Jauregui et al. 2015). A study on thermokarst lakes on the Qinghai-Tibetan Plateau
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